Embodiments of the invention are directed, in general, to wireless communication systems and, more specifically, to reference signal, also commonly referred to as pilot signal, transmission with wideband occupancy in multi-user wireless communications systems.
The disclosed invention considers as an exemplary embodiment a single-carrier frequency division multiple access (SC-FDMA) communications system as it is known in the art and further considered in the development of the 3GPP long term evolution (LTE). The invention assumes the uplink (UL) communication corresponding to the signal transmission from mobile user equipments (UEs) to a serving base station (Node B). The UEs scheduled for UL communication by the Node B (typically through control signaling in the downlink (DL) of the communication system, that is the communication from the Node B to UEs), are assumed to transmit a signal over a transmission time interval (TTI) corresponding to a sub-frame. This signal comprises of data and possibly of control information. In addition, in order to assist with the demodulation of the transmitted data, reference signals (RS), also known as pilots, are transmitted by each UE having an UL data transmission and occupy substantially the same bandwidth as the transmitted data signal. These demodulation RS will be referred to as DM RS.
FIG. 1 shows an exemplary sub-frame structure 110. The sub-frame comprises of four “short blocks (SBs)” 120 and twelve “long blocks (LBs)” 130 where the SB duration is practically half the LB duration. In the exemplary embodiment, the LB duration is assumed to be 66.67 microseconds (μsec). The DM RS is assumed to be transmitted in the SBs while the data is assumed to be transmitted in the LBs. However, this is only an exemplary generic setup and the 4 SBs may instead be replaced by 2 LBs in which case everything, including the DM RS, is transmitted in LBs. In addition to the time dimension, the transmission bandwidth (BW) is assumed to comprise of frequency resource units which will be referred to as resource blocks (RBs). UEs can be allocated a multiple N of consecutive RBs for their data transmission. In the example of FIG. 1, one LB RB is assumed to comprise of 12 sub-carriers 140 and one SB RB of 6 sub-carriers 150 (due to the fact that the SB duration is half the LB one). In addition, the LBs and SBs also include a cyclic prefix (not explicitly shown), as it is conventionally known for SC-FDMA and in general OFDM-based communication systems.
In order for the Node B to determine the RBs where a UE transmits its data signal and the modulation and coding scheme (MCS) used for the data transmission, a channel quality indicator (CQI) estimate is needed per RB of the scheduling BW or the total operating BW. The scheduling BW can be smaller than or equal to the total operating BW. This CQI estimate is obtained through the UE transmission of a wideband RS (CQI RS). The CQI RS (also referred to as sounding RS) may be transmitted in one or more LBs (CQI RS LBs) per multiple sub-frames replacing the data. As the CQI RS represents overhead, the CQI RS LB insertion rate should be minimized. FIG. 2 shows an exemplary structure with the CQI RS LB 210 inserted every 2 sub-frames 220, representing approximately 4.3% overhead. In addition to providing a CQI estimate, the CQI (sounding) RS may also serve for the purposed of transmission power control and transmission timing adjustments for the UEs.
The exemplary embodiment of the invention considers that the DM RS and CQI RS are constructed from a class of “Constant Amplitude Zero Auto-Correlation—CAZAC” sequences such as the Zadoff-Chu (ZC) sequences as proposed in TI-61162. An example for the construction CAZAC sequences are given by the following expression:
            c      k        ⁡          (      n      )        =            exp      [                                    j            ⁢                                                  ⁢            2            ⁢            π            ⁢                                                  ⁢            k                    L                ⁢                  (                      n            +                          n              ⁢                                                n                  +                  1                                2                                              )                    ]        .  In the above formula, L is the length of the CAZAC sequence, n is the index of a particular element of the sequence n={0, 1, 2 . . . , L−1}, and finally, k is the index of the sequence itself. For a given length L, there are L−1 distinct sequences, provided that L is prime. Therefore, the entire family of ZC sequences is defined as k ranges in {1, 2 . . . , L−1}. For ZC sequences of prime length L, the number of ZC sequences is L−1. Therefore, the larger the ZC length, the larger the number of such sequences, and the possible/easier the cell planning for allocating different ZC sequences to adjacent Node Bs for use by the DM RS. FIG. 3 illustrates the allocation principle of different ZC sequences (or, in general, CAZAC sequences) 310-370 to adjacent Node Bs. Alternatively, different ZC sequences can be respectively allocated to different cells of the same Node B in addition to different Node Bs.
In addition to using different ZC sequences, different cyclic shifts 410-440 of the same ZC sequence can be used to actually provide orthogonal RS in the code domain as proposed in TI-61162. This is illustrated in FIG. 4. In order for multiple RS generated from the same ZC sequence through correspondingly multiple cyclic shifts to be orthogonal, the cyclic shift value Δ450 should exceed the channel propagation delay spread D. For this reason, only a small number of cyclic shifts are possible, and if TLB is the LB duration, the number of cyclic shifts for the CQI RS is equal to the mathematical floor of the ratio TLB/D, assuming that the same cyclic shift value corresponding to a large delay spread is allocated to the CQI RS transmitted by all UEs regardless of the channel delay spread each such UE experiences.
In general, the larger the transmission BW of the CQI RS, the less accurate the CQI estimate becomes as the CQI RS power is spread over a wider BW. For UEs located near the cell edge that experience large propagation path losses in their transmitted signal, and therefore have low signal-to-interference and noise ratio (SINR) at the Node B receiver, this means large CQI inaccuracies if the CQI RS is transmitted over a wide BW. To improve the accuracy of the CQI estimate and the Node B scheduling decisions, the CQI RS from cell edge UEs should be typically transmitted over a smaller scheduling BW than the total available BW. For UEs located in the Node B interior, the opposite applies, and the CQI RS should be typically transmitted over the entire operating BW to maximize throughput gains from channel dependent frequency domain scheduling. Therefore, a mixture of CQI RS transmission BWs may need to be supported in a CQI RS LB.
As ZC sequences of different lengths are not orthogonal, CQI RS with different transmission BW, transmitted in the same CQI RS LB, need to occupy different sub-carriers (spectral combs) and therefore frequency division multiplexing (FDM) is applied. However, with FDM, the length the ZC sequence is typically short and therefore very few ZC sequences exist for allocation in neighboring Node Bs. For this reason, the multiplexing is typically a combination of FDM and CDM or pure CDM, with CDM referring to the use of different cyclic shifts of the same ZC sequence (code division multiplexing). The transmitter structures and corresponding CQI RS spectrums with CDM and FDM are shown in FIG. 5 and FIG. 6, respectively.
With the RS transmitter structure in FIG. 5, after the ZC sequence is generated 510, a cyclic shift (including a zero cyclic shift) may be subsequently applied 520. The cyclic shift may be prior to the Discrete Frequency Transform (DFT) operation 530 (as shown in FIG. 5) or after the Inverse Fast Fourier Transform (IFFT) operation 550 (the modification should be obvious as it involves just moving the corresponding block after the IFFT one). Subsequently, the time domain signal is converted to a frequency domain one through the application of a DFT operation 530 as it is known in the art. A pulse shaping filter may also be applied at the DFT output (not shown). The sub-carrier mapping operation 540 simply places the transmitted sub-carriers into the selected frequency band (RB) and is followed by the IFFT operation 550 and the Cyclic Prefix (CP) insertion 560. The reverse functions are performed at the receiver. The spectrum occupancy 570 is continuous.
The main difference between the transmitter structure in FIG. 6 and the one in FIG. 5 is that in the former the ZC sequence 610 (after the cyclic shift is applied 620) is repeated in time by a repetition factor (RPF) 630. Repetition in time produces in frequency a comb signal spectrum. The number of empty sub-carriers between two combs is equal to RPF minus one. Therefore, for RPF of 3, there will be two empty sub-carriers between two consecutive combs. If the CQI RS transmitted by all UEs occupies the same BW, no multiplexing of different BWs is necessary, and the transmitter structure in FIG. 5 may be used as it provides more ZC sequences and, due to continuous spectrum occupancy, it provides better immunity to interference as the CQI RS transmission power is spread over a larger number of sub-carriers. Similarly to the modification mentioned for FIG. 5, the comb spectrum may be generated directly in the frequency domain 650 and the cyclic shift may be applied after the IFFT 660.
When CQI RS from multiple UEs occupy more than one scheduling BWs, the multiplexing of orthogonal CQI RS is assumed to be achieved through a hybrid of CDM and FDM. CQI RS multiplexing through CDM is achieved through cyclic shifts of the same ZC sequence. Multiplexing of CQI RS through FDM is achieved by having the CQI RS occupy different combs of the spectrum. For example, UE transmitters as in FIG. 6 can be used to multiplex a number of orthogonal CQI RS equal to the number of cyclic shifts (for the same ZC sequence) providing immunity from the channel delay spread experienced from each CQI RS transmitted by UEs (CDM component). By providing a comb spectrum for the resulting CQI RS, other CQI RS can be transmitted in the unoccupied part of the spectrum (FDM component). The CQI RS occupying different parts of the spectrum may or may not use the same ZC sequence and may or may not occupy the same scheduling BW.
Based on previous discussion, it becomes apparent that there is a need for UEs to construct and transmit CQI (sounding) reference signal occupying a large bandwidth, not necessarily during a single transmission instance, in order to enable the serving Node B to perform frequency selective channel dependent scheduling in the uplink of wireless communication systems.
There is another need for the serving Node B to signal to the UEs the bandwidth over which to transmit the CQI (sounding) reference signal.
There is another need for the serving Node B to signal to the UEs parameters required for multiplexing the CQI (sounding) reference signals from a plurality of UEs over the same transmission bandwidth.
There is another need for the serving Node B to power control the transmission of CQI (sounding) reference signals as they may constitute interference to adjacent cells or Node Bs.
There is another need to randomize the interference experienced by the CQI (sounding) reference signals in order to provide an accurate measure for the estimated channel response.
There is another need to co-ordinate orthogonal transmission of CQI (sounding) reference signals from multiple UEs.